The present invention is directed toward semiconductor transistors, and more particularly to complementary lateral bipolar junction transistors (BJTs) and methods for fabrication such transistors.
From the mid to the late 1970's, there was a lot of excitement in the VLSI industry about the prospect of Integrated Injection Logic (I2L). I2L is by far the densest circuit. It uses small sized devices, and requires one PNP per gate for current injection and only one NPN per fan-out. Thus an inverter with FO=3 takes only four transistors. The NPN transistors in an I2L circuit operate in the reverse-active mode. As a result, even with advanced self-aligned vertical Si-based BJT technology, I2L has minimum delays not much below one ns. This speed limitation, together with the rapid progress in complementary metal-oxide semiconductor (CMOS) scaling in the early 1980's, caused the demise of I2L.
Digital logic is currently dominated by silicon CMOS circuits. It is desirable to reduce the operating voltage for CMOS circuits due to increased power consumption and heating in scaled CMOS technologies. However, CMOS performance is reaching a limit due to its poor signal-to-noise margins at low operating voltages (i.e., less than 0.5 volts).
In a BJT inverter circuit, the output current is exponentially dependent on the input voltage, giving much higher transconductance and potentially faster switching speed than CMOS. However, conventional vertical BJTs are generally not suitable for high density digital logic because of their large footprint due to isolation structure, their large parasitic capacitance due to the relatively large base-collector junction area, and associated minority carrier charge storage when biased in the saturation mode, that is when the collector-base diode is forward biased.
In contrast to vertical BJTs, when a lateral NPN transistor is turned on with a voltage VBE, its base current flows vertically down from the base terminal and then turns and flows in the intrinsic base horizontally toward the emitter. The vertical base current flow causes a vertical IR drop between the top (p+/p interface) and the bottom (p/BOX interface) of the intrinsic base, causing V′BE (top) to be larger than V′BE (bottom). When this voltage difference is larger than kT/q, there is appreciable current crowding, with the local current density appreciably larger near the top than near the bottom. As the current increases, at some point the local minority-carrier density becomes larger than the majority-carrier density. Beyond that point, the dependence of current on VBE degrades. For a vertical transistor, this “current degradation” point is determined by the collector, which is the most lightly doped region. For a symmetric lateral transistor, the degradation point is determined by the base, which is the most lightly doped region.
In complementary thin-base symmetric lateral BJTs on SOI, there is the absence of a lightly doped collector. The lateral transistors have no deleterious base push out effect. Unlike vertical BJTs, there is no rapid performance drop off at high current densities. The lateral transistors operate equally fast in forward-active and reverse-active modes. The unique characteristics of symmetric lateral BJTs, with no base push out and equal speed in forward-active and reverse-active modes, suggest a need to rethink BJT circuits and circuit opportunities offered by the technology.
The layout of lateral BJT is similar to that of CMOS. One difference could be in the placement of metal contact to the extrinsic base. In CMOS, the metal contact to the gate is located away from the inversion channel region. In a BJT, if rbX is a concern, metal contact to the extrinsic base should be located over the intrinsic base, not away from the intrinsic base. If needed, two silicon thicknesses may be used, one for CMOS and one for lateral BJT.
BJTs (also referred to as bipolar transistors) are used in driver circuits where high current is required given the exponential dependence of the output current on the input voltage. On the other hand, BJTs are limited by the need of relatively large input voltage to achieve the target current level. The turn-on voltage of BJTs is dependent on the bandgap of the consisting materials. For BJTs made of Si, a typical turn-on voltage of approximately 0.9V to 1V is needed.